1. The Field of the Invention
The present invention relates generally to x-ray tube devices. In particular, embodiments of the present invention relate to a cooling system for stationary anode x-ray tubes that employs extended surfaces to increase the rate of heat transfer from the x-ray tube so as to significantly reduce heat-induced damage within the x-ray tube structure and thereby extend the operating life of the device and permit operation of the x-ray tube device at relatively higher power settings than would otherwise be possible.
2. Prior State of the Art
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. Such equipment is commonly used in applications such as diagnostic and therapeutic radiology, semiconductor manufacture and fabrication, and materials testing. While used in a number of different applications, the basic operation of x-ray tubes is similar. In general, x-rays, or x-ray radiation, are produced when electrons are produced, accelerated, and then impinged upon a material of a particular composition.
Regardless of the application in which they are employed, these devices typically include a number of common elements including a cathode, or electron source, and an anode situated within an evacuated enclosure in a spaced apart arrangement. The anode includes a target surface oriented to receive electrons emitted by the cathode. In operation, an electric current applied to a filament portion of the cathode causes electrons to be emitted from the filament by thermionic emission. The electrons thus emitted then accelerate towards a target surface of the anode under the influence of an electric potential applied between the cathode and the anode. Upon approaching and striking the anode target surface, many of the electrons either emit, or cause the anode to emit, electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Anode target surface materials with high atomic numbers (xe2x80x9cZxe2x80x9d numbers) are typically employed. The x-rays are then collimated so that they exit the x-ray tube through a window in the tube, and enter the x-ray subject. As is well known, the x-rays can be used for therapeutic treatment, x-ray medical diagnostic examination, or material analysis procedures.
As discussed above, some of the electrons that impact the anode target surface convert a substantial portion of their kinetic energy to x-rays. Many electrons, however, do not produce x-rays as a result of their interaction with the anode target surface, but instead impart their kinetic energy to the anode and other x-ray tube structures in the form of heat. As a consequence of their substantial kinetic energy, the heat produced by these electrons is significant. Still other electrons simply rebound from the target surface and strike other xe2x80x9cnon targetxe2x80x9d surfaces within the x-ray tube. These are often referred to as xe2x80x9cbackscatterxe2x80x9d electrons. These backscatter electrons retain a significant amount of kinetic energy after rebounding, and when they impact these other non-target surfaces, heat is generated within the x-ray device. The heat generated as a consequence of electron impacts on the target surface and other x-ray device structures must be reliably and continuously removed. If left unchecked, it can ultimately damage the x-ray tube and shorten its operational life.
Some x-ray generating devices at least partially alleviate this heat problem by employing an anode that continuously rotates within the device. This rotation distributes the heat over a larger area of the anode, allowing for more efficient dispersal of heat in the x-ray tube and reducing the chances of heat damage to the device. However, some applications such as x-ray fluorescence and spectrometry in sample analysis, and product and process control in the metals and cement industries, are best performed using stationary anode x-ray generating devices. Thus, alternative approaches to cooling have been developed for use with these types of devices.
One such approach involves the use of a cooling fluid circulated within the x-ray device. An example of this approach involves circulating a cooling fluid through a passageway formed within the interior of the anode so as to remove heat conducted to the anode from the anode target surface. This process is sometimes referred to as xe2x80x9cimpinging flow heat transferxe2x80x9d because at least a portion of the coolant flow is caused to impinge upon, or impact, at least one of the surfaces or structures of the x-ray tube from which heat is to be removed. This approach has proven problematic in some instances however, primarily due to the cooling fluids typically employed.
A variety of cooling fluids have been used in such a stationary anode x-ray generating device cooling system. Due to the structural and operational characteristics of the x-ray device, the cooling fluid employed must possess certain characteristics. For example, the cooling fluid must have an acceptable thermal efficiency, i.e., be capable of effectively absorbing and removing the significant heat produced during operation of the x-ray device. Furthermore, the high electric potential between the cathode and the anode necessitates the use of a cooling fluid that is electrically non-conductive, or xe2x80x9cdielectric.xe2x80x9d
Various dielectric coolants have been employed in the context of stationary anode x-ray devices. For example, deionized water has been found to be an acceptable cooling fluid in some stationary anode x-ray generating devices because of its efficient heat absorption capabilities and non-conductivity. However, deionized water must be constantly monitored and processed to ensure that it retains its dielectric property. Such monitoring and processing increases the cost and complexity of the x-ray device cooling system. In view of the disadvantages of deionized water as a cooling fluid, alternative fluids have been utilized. For example, dielectric oils are commonly employed in stationary anode x-ray generating devices because of their non-conductivity. Further, they are somewhat more desirable than deionized water in that they do not require maintenance or processing to maintain their nonconductive properties.
While dielectric fluids are generally desirable cooling media for x-ray device applications due to their electrical properties, they have proven unable to adequately cool many stationary anode x-ray devices. Thus, a need exists for improving the rate of heat transfer that is currently achieved in typical stationary anode x-ray tubes.
The present invention has been developed in response to the current state of the art, and in particular, in response to these and other problems and needs that have not been fully or adequately solved by currently available stationary anode cooling systems. Thus it is an overall object of embodiments of the present invention to resolve at least the aforementioned problems and shortcomings in the art by providing an x-ray tube cooling system that facilitates a relative increase in the rate at which heat is transferred from x-ray tubes. Embodiments of the present invention are especially well-suited for use in the context of stationary anode x-ray tubes. However, it will be appreciated that the features and advantages of the present invention may find useful application in other types of x-ray devices as well
Briefly summarized, the foregoing objects and advantages are provided by an x-ray tube cooling system employing a surface area augmentation structure having a plurality of extended surfaces configured to transfer heat from the stationary anode and other x-ray tube structures to a liquid coolant circulating through the stationary anode.
In a preferred embodiment, the surface area augmentation structure comprises a cooling disk having an annular body defining an aperture, and a plurality of cooling fins disposed about the aperture at regular intervals and extending from the annular body. Preferably, the cooling fins are integral with the annular body. The cooling disk is disposed within a fluid passageway partially defined by the anode so that the cooling disk is in substantial contact with both the anode and coolant flowing through the fluid passageway.
In operation, an external cooling unit produces a flow of coolant that is continuously circulated through coolant supply and coolant return passageways. The coolant leaving the external cooling unit is introduced into the anode by way of a coolant injection assembly. The coolant injection assembly includes a nozzle at the downstream end so that coolant exiting the coolant supply passageway of the coolant injection assembly is caused to accelerate as it exits the coolant supply passageway. After exiting the coolant supply passageway, the rapidly moving coolant flows towards the cooling disk disposed proximate to the nozzle. Upon reaching the cooling disk, the coolant passes through the aperture defined by the annular body of the cooling disk. The cooling fluid then exits the cooling disk aperture and impinges upon a flow diverter disposed inside the anode opposite the cooling disk.
Preferably, the flow diverter is integral with the anode. The flow diverter serves both to direct the coolant flow exiting the disk into the coolant return passageway and to transmit heat from at least the anode to the coolant passing through the coolant disk.
After being redirected by the flow diverter, the cooling fluid then passes between the fins of the cooling disk and, by so doing, absorbs heat conducted to the cooling disk from the anode. The cooling fluid is then conveyed via the coolant return passageway back to the external cooling unit where it is cooled before reentering the coolant injection assembly and repeating the cycle. Because of the surface area augmentation employed in the cooling system of the present invention, heat is conducted away from the x-ray tube in a substantially more efficient manner than would otherwise be the case. This increased rate of heat transfer prolongs the life of the x-ray device and allows for greater operational flexibility. Further, the use of an impinging coolant flow in conjunction with the flow diverter results in highly efficient convective cooling of the anode and other x-ray tube structures.
These and other objects and features of the present claimed invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.